1. The Field of the Invention
The present invention relates to a glass ceramic plate providing a cooking surface of a cook top or cooking apparatus, which is composed of a glass ceramic material which is transparent to visible light as well as IR radiation and which is provided with a coating on an underside thereof in the form of a noble metal film. The invention also relates to a coating on an underside of the glass ceramic plate. The present invention relates to a process for coating the underside of the glass ceramic panel.
2. Related Art
Glass ceramic plates, which are used in cooking devices to provide cooking surfaces, so-called glass ceramic cooking surfaces, are typically darkly colored in the melt for the European market, so that they appear black when viewed from the top and prevent viewing of components in the interior of the cooking devices. Generally under intense illumination from above, especially in the modern kitchens, e.g. under conventional halogen lighting, cables or other components in the interior of the cooking devices may still be visible from above. In order to provide an outstandingly opaque cooking surface, an opaque silicone coating is applied to the typically knobbed underside of the glass ceramic plate. The opaque silicone coating must be generally omitted in the display area and in the vicinity of the light of the residual-heat signaling device, so that the light indicator signals are visible to the user. Generally the knobbed structure on the underside of the glass ceramic plate in the vicinity of the light indicator devices (display area and residual heat indicator) is dressed with a smooth colorless silicone coating, so that the observable signals are not distorted.
Currently colorless glass ceramic plates, i.e. glass ceramic plates that are not colored in the melt, which are transparent for visible light and smooth on both sides, are widely used to provide cooking surfaces, especially in Japan. These glass ceramic plates are coated on their undersides in a special way, so that the cooking apparatus interior cannot be viewed from above through them. The cooking surfaces on the colorless smooth glass ceramic plates have the advantage that changing the coatings on the undersides of the glass ceramic plates can change their color in a simple manner. Thus the same colorless glass ceramic plate can be made to appear silver, yellow, green, or any other color besides black by means of the underside coating. Because this glass ceramic plate has no knobs on its underside illuminated cooking zone indicators, displays, or residual heat indicators can be directly mounted on the cooking surface underside and are sufficiently visible, in as much as the opaque coating is omitted in these areas.
Noble metal films used as underside coatings are described in Japanese Disclosure Document H7-17409. Furthermore EP 1,267,593 B1 describes an underside coating based on a glass flux and an inorganic pigment, which can be provided with an additional coating based on organic compounds (silicones, polyamides, among others). DE 100 14 373 C2 mentions sol-gel coatings besides noble metal coatings. Sputtered coatings are mentioned in WO 03/098115 A1.
These coatings have several disadvantages as underside coatings for glass ceramic plates that provide cooking surfaces, as shown by the following results described hereinbelow.
The noble films described in JP H7-17409 are not resistant to burned food because of their content of transition metals, e.g. silver. However the resistance of the underside coating to food is necessary for underside coatings in cooking devices with gas burners, since the food can reach the underside of the glass ceramic plate providing the cooking surface through the openings in the glass ceramic plate for the gas burners, which are necessary for operation of the gas burners. The known solutions are not suitable for a gas cooking apparatus.
Since the noble metal films described in JP H7-17409 are lustrous metal films and reflect light, defects or faults in the glass ceramic (e.g. small bubbles, scratches, or crystalline inclusions or stones) are reflected, i.e. doubled, by these noble metal layers and they are thus especially clearly visible. However small defects in the glass ceramic, such as scratches or bubbles, cannot be completely avoided during production of colorless glass ceramic plates that provide cooking surfaces, so that the exclusive coating of colorless glass ceramic plates with a noble metal preparation leads to a high rejection rate and thus to an uneconomical process.
Porous underside coatings made of glass flux and inorganic pigments according to EP 1,267,593 B1 or based on sol-gel methods described in DE 100 14 373 C2 have similarly proven to be unsuitable for use in gas cooking appliances, because food reaching the undersurface of the glass ceramic plate already forms clearly visible flecks without temperature treatment. Likewise no permanent protection could be obtained by using the sealing silicone coating, as proposed in the above-mentioned document, since the silicone coating cannot withstand the higher temperature in the vicinity of the gas burners (about 450 to 550° C.). Also the destruction of the silicone layer leads to a definite color change (a brightening) of the underside coating.
Sputtered coatings according to the WO reference—even when they were sufficient for the chemical resistance and temperature resistance requirements—have the disadvantage that an expensive marking engineering, e.g. according to JP(A) 2004 342 609, is required for forming display windows or other openings for light signals.